1. Field of the Invention
The present invention relates to watermark embedment and extraction, and more particularly, to a method for blindly embedding and extracting a watermark by using wavelet transform and a human visual system (HVS) model, which obtains both robustness and invisibility. In the method, the HVS model imitating a human visual system is applied to a middle frequency band for wavelet transformation and a quantization step determined adaptively according to the importance of wavelet coefficient is used.
2. Description of the Related Art
As a user is charged for using digital contents, it has been actively studied to protect the copyright of the digital contents. The digital watermarking technique is the most widely used copyright protection technique in which information on a copyright holder is embedded into the digital contents but is not recognized by a human eye. Accordingly, the copyright holder can prove to hold a copyright or an ownership of the digital contents by extracting the copyright information from the digital contents when the digital contents are used or the copyright dispute occurs.
To achieve the purpose described above, the digital watermarking technique should have properties such as invisibility, robustness and tolerant error detection rate. The invisibility means that the embedded watermark cannot be easily recognized by a human eye. The robustness means that the embedded watermark is not destroyed or changed in spite of intended external conversion, lossy compression, various image processes and noises. However, since the invisibility and the robustness have tradeoff relation to each other, one of the most important aims of the watermarking technique is to obtain the robustness as well as the invisibility with minimizing degradation of image quality. Therefore, it has been actively studied to obtain both the robustness and the invisibility.
On the other hand, there are two kinds of watermarking techniques: a blind watermarking technique and a non-blind watermarking technique. In the blind watermarking technique, the watermark is extracted from the digital contents without any original data. In the non-blind watermarking technique, the watermark is extracted from the digital contents by using the original data.
The blind watermarking technique does not need any additional storage for the original data. Recently, the blind watermarking technique is actively studied because of its availability for Certification Authority. However, since the blind watermarking technique does not have any reference data for watermark extraction, it is available but is not robust against an attack.
Accordingly, the blind watermarking technique has been studied considering a data compression attack and an image processing attack such as filtration, especially a geometrical attack that is counted as a very strong attack to the blind watermarking technique.
Most of the techniques against the geometric attack are the watermarking technique for embedding a watermark into the digital contents in a frequency domain by using transforms such as DCT, DFT and DWT.